1) Field of the Invention
The present invention relates to apparatuses and methods for forming ducts and, more specifically, thermoplastic ducts formed by consolidation joining.
2) Description of Related Art
Ducts provide transport passageways for a wide variety of applications. For example, tubular ducts are widely used for air flow in aircraft environmental control systems. Similarly, ducts provide passageways for transporting gases for heating and ventilation in other vehicles and in buildings. Water distribution systems, hydraulic systems, and other fluid networks also often use ducts for fluid transport. In addition, solid materials, for example, in particulate form can be delivered through ducts. Ducts for the foregoing and other applications can be formed of metals, plastics, ceramics, composites, and other materials.
One conventional aircraft environmental control system utilizes a network of ducts to provide air for heating, cooling, ventilation, filtering, humidity control, and/or pressure control of the cabin. In this conventional system, the ducts are formed of a composite material that includes a thermoset matrix that impregnates, and is reinforced by, a reinforcing material such as Kevlar®, registered trademark of E. I. du Pont de Nemours and Company. The thermoset matrix is typically formed of an epoxy or polyester resin, which hardens when it is subjected to heat and pressure. Ducts formed of this composite material are generally strong and lightweight, as required in many aircraft applications. However, the manufacturing process can be complicated, lengthy, and expensive, especially for specially shaped ducts such as curved ducts and ducts that include a spud or attached fitting, a bead, a bell or flared portion, a conical section, or another contour. For example, curved ducts are conventionally formed around a disposable plaster mandrel. The plaster mandrel is formed in a specially shaped rotatable tool that acts as a mold to form the plaster mandrel according to the desired shape of the duct. First, a cavity of the tool is partially filled with uncured plaster, and the tool is rotated so that the plaster coats an inner surface of the tool cavity. When the plaster is partially cured to form the mandrel, the tool is stopped and opened so that the plaster mandrel can be removed and placed in an oven for subsequent curing. The mandrel is then treated with a sealant, cured again, and treated with a release agent. Plies of fabric, such as Kevlar®, preimpregnated with the thermoset material are cut and draped over the mandrel, often by hand, and a heat gun is used to mold the plies to the shape of mandrel. The mandrel is placed in a vacuum bag, which is fitted with one or more valves, and air is evacuated from the bag through the valves so that the bag urges the plies against the mandrel and consolidates the plies while heat is applied to cure the plies and form the duct. When the plies are cured, the vacuum bag is removed and the plaster mandrel is broken and removed from the duct. The duct is cleaned and trimmed to the desired dimensional characteristics. One or more jigs that correspond to the desired shape of the duct are often used for trimming the duct and for accurately locating additional features on the duct such as holes, spuds, brackets, and the like. Further processing is sometimes necessary for adding a bead or bell so that one or both ends of the duct can be secured and sealed to another duct. Typically, a bead is formed by adding additional material, thus adding weight to the duct. Insulation can also be added to the inside and/or outside of the duct.
The manufacturing process for such reinforced thermoset ducts is complicated, time consuming, and expensive. The rotatable tool used to mold the plaster mandrel is specially sized and shaped for creating a duct of specific dimensions, so numerous such tools must be produced and maintained for manufacturing different ducts. The plaster mandrel is formed and destroyed during the manufacture of one duct, requiring time for curing and resulting in plaster that typically must be removed or destroyed as waste. Additionally, the preimpregnated plies change shape while being cured and consolidated and therefore typically must be trimmed after curing to achieve the desired dimensions. The jigs required for trimming and for locating the proper positions for features such as holes and spuds are also typically used for only a duct of particular dimensions, so numerous jigs are required if different ducts are to be formed. Like the rotatable tools used for forming the mandrels, the jigs require time and expense for manufacture, storage, and maintenance.
Additionally, ducts formed of common thermoset epoxies do not perform well in certain flammability, smoke, and toxicity tests, and the use of such materials can be unacceptable if performance requirements are strict. For example, changes in environmental laws or proposed changes to performance requirements mandated by the Federal Aviation Administration would prevent the use of ducts formed from some thermoset composites in certain aircraft environmental control system applications.
One proposed alternative to thermoset composite materials is thermoplastic composites. Thermoplastic composites become plastically deformable when heated above a glass transition temperature. Instead of laying plies of uncured composite material on a mandrel, a sheet of thermoplastic composite material can be manufactured and then heated and formed to a desired shape. Thus, a part can be formed from a thermoplastic composite without using a disposable plaster mandrel and a special tool for forming the mandrel.
The formation of certain shapes of parts, such as ducts, from thermoplastic composite materials requires the formation of joints. Methods for joining members formed from thermoplastic composites are known in the art, but none of the known methods are ideal. Generally, each method of joining thermoplastic composite members includes heating the members to a temperature above the glass transition temperature and holding the members together. One method of providing heat to the members is by generating friction between the members, for example, by reciprocating, ultrasonically vibrating, or friction stirring the members. Undesirably, composites that contain fiber reinforcements, especially long or continuous fibers, can be damaged by these frictional heating methods. Locating tools and backing members for supporting the members are often required, and large members can be difficult to reciprocate. Additionally, ultrasonic methods typically require surface preparations, and friction stirring is typically slow.
Alternatively, heat can be applied by conduction or convection, for example, by hot plate joining, hot gas joining, extrusion joining, or resistance joining. In hot plate joining, a plate is heated and inserted at an interface of the members. The plate is then removed and the members are pressed together. Hot plate joining generally requires simple tooling but is time consuming and is not practical for use with complex shapes. Further, the hot plate can introduce contamination into the interface of the members or oxidize the composite materials, thereby weakening the joint. Hot gas joining is similar to conventional metal welding. An operator inserts a filler rod, typically formed of the composite material, into the interface and directs a stream of hot gas to heat the members and the rod. The gas plasticizes the members and the rod, which provides additional material into the interface. Similarly, extrusion joining is performed by heating the filler rod in an extruder and extruding the heated rod material into the interface while using the hot gas to heat the members. Hot gas and extrusion joining are typically slow, and the quality of the resulting joint can vary significantly depending on the skill of the operator. In resistance joining, an electrically conductive heating element is inserted into the interface. The members are pressed together, and the heating element is electrically energized, causing resistive heating therein, which heats the members. The heating element, which remains in the joint, increases the cost of the joining method and affects the characteristics of the joint, for example, making the joint stiffer than the other portions of the members. Typically, the heating element has a different coefficient of thermal expansion than the thermoplastic material, resulting in stresses in the joint when heated or cooled.
Finally, heat can be provided to the interface by electromagnetism, for example, by electromagnetic joining, microwave joining, laser joining, and infrared joining. Electromagnetic joining is accomplished by dispersing a metallic powder in a bonding material in the interface of the members to be joined. A magnet is moved proximate to the interface, thereby generating heat in the powder. The powder adds to the cost of the joint, and the method is generally limited to joining members of limited thickness. Where a first member has a low absorption and a second member has a high absorption, laser joining can be used by directing a laser beam through the first member so that it is absorbed at the interface by the second member. Laser joining is generally not applicable where the members do not have dissimilar absorptions. In microwave joining, a material susceptible to microwaves is placed in the interface, and the interface is irradiated with microwaves. The method is typically used only if the members are not significantly absorptive of microwaves. Infrared joining, i.e., using an infrared lamp to heat the interface and then pressing the members together, requires a complicated set up and can be time consuming, depending on the absorption characteristics of the members.
Thus, there exists a need for an improved apparatus and method of forming ducts that is effective and cost efficient. Preferably, the method should not require that individual plies be laid on a plaster mandrel. The method should be compatible with plastic and composite materials that provide high strength-to-weight ratios and meet strict flammability, smoke, and toxicity standards. Further, the method should provide a method of forming strong joints and should be adaptable for automated operation to achieve consistent results.